The Collaborative International Dictionary of English v.0.48:

Warning \Warn"ing\, a.
   Giving previous notice; cautioning; admonishing; as, a
   warning voice.
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         That warning timepiece never ceased.     --Longfellow.
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   Warning piece, Warning wheel (Horol.), a piece or wheel
      which produces a sound shortly before the clock strikes.
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The Collaborative International Dictionary of English v.0.48:

Warning \Warn"ing\, n.
   1. Previous notice. "At a month's warning." --Dryden.
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            A great journey to take upon so short a warning.
                                                  --L'Estrange.
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   2. Caution against danger, or against faults or evil
      practices which incur danger; admonition; monition.
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            Could warning make the world more just or wise.
                                                  --Dryden.
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The Collaborative International Dictionary of English v.0.48:

Warn \Warn\, v. t. [imp. & p. p. Warned; p. pr. & vb. n.
   Warning.] [OE. warnen, warnien, AS. warnian, wearnian, to
   take heed, to warn; akin to AS. wearn denial, refusal, OS.
   warning, wernian, to refuse, OHG. warnen, G. warnen to warn,
   OFries. warna, werna, Icel. varna to refuse; and probably to
   E. wary. ????.]
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   1. To make ware or aware; to give previous information to; to
      give notice to; to notify; to admonish; hence, to notify
      or summon by authority; as, to warn a town meeting; to
      warn a tenant to quit a house. "Warned of the ensuing
      fight." --Dryden.
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            Cornelius the centurion . . . was warned from God by
            an holy angel to send for thee.       --Acts x. 22.
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            Who is it that hath warned us to the walls? --Shak.
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   2. To give notice to, of approaching or probable danger or
      evil; to caution against anything that may prove
      injurious. "Juturna warns the Daunian chief of Lausus'
      danger, urging swift relief." --Dryden.
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   3. To ward off. [Obs.] --Spenser.
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WordNet (r) 3.0 (2006):

warning
    adj 1: serving to warn; "shook a monitory finger at him"; "an
           exemplary jail sentence" [syn: admonitory,
           cautionary, exemplary, monitory, warning(a)]
    n 1: a message informing of danger; "a warning that still more
         bombs could explode"
    2: cautionary advice about something imminent (especially
       imminent danger or other unpleasantness); "a letter of
       admonition about the dangers of immorality"; "the warning was
       to beware of surprises"; "his final word of advice was not to
       play with matches" [syn: admonition, monition, warning,
       word of advice]
    3: notification of something, usually in advance; "they gave
       little warning of their arrival"; "she had only had four
       days' warning before leaving Berlin"

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "warning":
   admonishing, admonition, admonitory, advice, advising, advisory,
   advocacy, alerting, augural, blackmail, briefing, bulldozing, call,
   call for, caution, cautionary, cautioning, caveat, claim, clue,
   commination, consultation, consultative, consultatory,
   contribution, council, counsel, cue, demand, demand for,
   denunciation, determent, deterrence, deterrent, didactic,
   direction, directive, draft, drain, duty, empty threat, exaction,
   exemplary, exhortation, exhortative, exhortatory, expostulation,
   expostulative, expostulatory, extortion, extortionate demand,
   foreboding, forerunning, foreshadowing, foreshowing, foretokening,
   forewarning, frightening off, guidance, heavy demand,
   heavy with meaning, hint, hortation, hortative, hortatory, idea,
   idle threat, imminence, implied threat, imposition, impost, indent,
   indicative, insistent demand, instruction, instructive,
   intimidation, intuitive, levy, meaningful, menace, monition,
   monitorial, monitory, moralistic, nonnegotiable demand, notice,
   notificational, notifying, office, opinion, order, parley,
   passing word, pointer, preachy, precursive, precursory, predictive,
   prefigurative, preindicative, premonitory, presageful, presaging,
   prognostic, prognosticative, promise of harm, proposal,
   recommendation, recommendatory, remonstrance, remonstrant,
   remonstrative, remonstratory, requirement, requisition, rush,
   rush order, sententious, significant, steer, suggestion,
   sword of Damocles, talking out of, tax, taxing, thought, threat,
   threateningness, threatfulness, tip, tip-off, tribute, ultimatum,
   whisper